A geographical history of Earth across deep time
As humanity is waiting on the wings, about to make her leap onto the foreground for these last stages of the Great Chain, it is worthwhile to evoke the evolution of the earth’s geographical features. That theme is to be distinguished from the geological development of our planet, a natural process that is best envisaged in its full generality. As we shall see in due time, geography is a study of location, the sole stage of the human theatre to date, rather than its geological underpinnings.
The continents
We have explored earlier the basics of plate tectonics: rigid areas of the lithosphere “float” over the viscous asthenosphere in a long-winded ballet that shall only end when the earth is finally tapped out of its geologic vitality and turns into inert rock. Until then, the dance of the plates, and in particular the constant dispersal and re-assembly of the planet’s continental crust, will delineate the most striking geographical feature of all: the continents.
The term "dance" is not a convenient simile. The tectonic cycle seems to follow a rythmic pattern, with four to six hundred million years between beats. At times, most of the world’s continental crust is gathered into a single landmass called a supercontinent, before being torn asunder: one plate rifts apart, eventually becoming a continent; a young sea opense, eventually maturing into an ocean basin. Later, somewhere else, a basin is closed when the bordering contents collide, raising a mountian belt and sealing a new suture.
Although that sequence is more inferred than known, we now believe there have been six supercontinent cycles since the first one, Ur, assembled from disparate cratonic fragments (primordial continental crust) three billion years ago. After Ur came Kenorland, Columbia, Rodinia and Panottia, each one around half a billion years after the other (on average). Ur may have been as small as current Australia, with the ratio of continental crust increasing with repeated subductions, although it is not clear that trend is continuing now (erosion and other phenomena also create oceanic crust, suggesting a possible equilibrium).
Pannotia fractured into Gondwana, Laurentia, and the smaller Baltica and Siberia. About 300 million years ago, successive accretion led to a single C-shaped supercontinent called Pangea, surrounde by a global ocean (Panthalassa) and embracing a smaller sea, Thetys on the equator.
The breakdown of Pangea progressively gave birth to the current system of tectonic plates, and to the continents we know. Starting with the tectonic plates, seven are considered major plates. In decreasing order of area they are: the Pacific, Antarctic, North American, Eurasian, African, Australian and South American plates. Together, they cover 457 of the total 510 million square kilometers of Earth’s surface (90%). The largest minor plates are two continental plates that have hit and merged with Eurasia: the Indian and Arabian plates; two oceanic plates, Nazca and Philippine, on either side of the Pacific; and the Somali plate, a mixed one that includes the Horn of Africa, Madagascar and part of the Indian Ocean. Less well-known is the existence of thousands of micro-plates, forming a quasi-fractal structure, which makes the tectonic system a complex one at different scales.
The idea of continent is less clear. What seems like a pre-requisite is to be a large, continuous landmass. Yet, the underlying idea is also that of a clean partition of all the globe’s land. Islanders may talk of "the continent" as something distinct from their abode, but every island in the world is assigned to a continent in various administrative typologies, and "Oceania" exists solely to cast a wide net and capture all the "uncategorized".
Should we focus on continous landmasses, the largest one, Afro-Eurasia, counts for almost half of the earth’s land (Eurasia and Africa are connected by land at the isthmus of Suez). America is the second largest (the North and South halves are connected in the Darién mountains on the isthmus of Panama), followed by Antarctica and Australia (the top four represent 90% of all land). Next come Greenland, New Guinea, Borneo, Madagascar, and of course scores of others.
To that purely geographical approach we can adjoin a geological notion. Let’s remind ourselves of difference between continental crust and oceanic crust. The latter, being thinner and denser, sits lower and tends to act as a well for the earth’s water. That is not a rule however: continental crust cannot be equated with land, nor oceanic crust with a body of water.
Some parts of the continental crust can be submerged under water, in which case it is called the continental shelf. Most of the world’s islands are continental islands: sections of continental crust that emerge from the water in the midst of a continental shelf. Greenland is part of the same continuous continental crust extent as the rest of North America.
Conversely, some parts of the oceanic crust form landmasses called oceanic islands, typically borne out of volcanic hotspots or coral reef growth. Famous examples include Hawaii or the Galapagos islands.
All in all, there will be some judgment in any list of continents. Given the identification of plate tectonics with "continental drift", it would not make sense to consider multi-plates landmasses. We can then stick to the non-Pacific major plates, alongside their continental shelves (so we properly capture the continental islands. That would be: Eurasia, Africa, North America, South America, Australia, Antarctica. That is pretty much the traditional list, except for Europe, which has no reason other than conventional and historical to exist as anything else than a Eurasian peninsula; and for Oceania, a geographical amalgam that capture large swathes of oceanic crust — while it makes sense to restrain ourselves to continental crust.
There are more continuous landmasses with their own tectonic plate than those six. That said, we have seen there are hundred of micro-plates, and surely many of those have micro-islands on their territory, so we need some kind of arbitrary cutoff. We will only mention two cases: India and Arabia, both of definitely used to be distinct continents, but that have hit Eurasia so hard, embedding themselves into it for the long run, that they are often called subcontinents at most.
If we do not insist on large landmasses, we can also observe submerged continents: large swathes of continental crust that is mostly underwater except for some islands. Zealandia, a large portion of ocean surrounding the islands of New Zealand, has been recognized as an official continent. However it does not have its own tectonic plate and is located at the junction of the Australian and Pacific ones.
The poles
The earth’s polar regions have not always been icehouses, as used as we are to see their pristine white color in space photography or human representations.
Antarctica has been centered around the South Pole even before it was a proper continent, when it was still part of Gondwana. During that era, it was thermally insulated by ocean currents; dinosaur fauna roamed its green valleys and forests of conifers. Australia was the first subcontinent to break away, but it is Antarctica’s separation from South American which changed its fate, around 40 million years ago. When the rift was complete, the Drake passage opened, becoming a gateway for the Antarctic Circumpolar Current, which encircled the continent in a ring of cold water, severing it from the rest of the world. Glaciers formed in the Transantarctic mountains and progressively descended until the whole continent was covered by an ice sheet. Today, it stores 70% of the planet’s fresh water.
The North Pole is not covered by a continent, but sits above an ocean basin, nearly enclosed by the surrounding landmasses of Eurasia and North America: the Arctic ocean. Permanent sea ice is only a few million years old. Fifty million years ago, the Arctic was a brackish, lake-like sea circled with forests and inhabited by alligators. The following glciation is only a product of the Quaternary ice ages.
The seas
As already mentioned, not only is the earth the only known world covered with water, about 73% of its surface is submerged. The largest continuous body of water is the World Ocean, which represents 71 out of that 73%. The remaining 2% is made up of numerous lakes (closed bodies of water inside a continent or an island).
Conventionally, the world Ocean is divided into several individual oceans.
The Pacific is the senile remnant of Panthalassa, shrinking in size but still representing about half of the World Ocean.
The Atlantic is a young and still-widening ocean, borne out of Pangaea’s breakup.
The Indian Ocean took shape as India, Madagascar, Australia, and Antarctica broke away from their Gondwanan parent.
The Southern Ocean is not always formally recognised as such; it is defined by the circumpolar current that courses around the Antarctic
The Arctic ocean is the smallest and shallowest, an almost-landlocked basin whose seasonal ice cap connects in winter to the surrounding shelves.
Within and between these oceans lies a complex geography of seas, gulfs, bays, straits. They can be distinguished from the oceans by the shape of landforms they border, by prevalent currents, or sometimes on a purely conventional basis by borders or coordinates.
Around the Pacific, the most notable are the Philippine Sea, the Coral Sea, the South China Sea, the East China Sea, the Tasman Sea, the Sea of Okhotsk, and to the north the Bering Sea and the Gulf of Alaska. For the Atlantic, we can mention the Mediterranean Sea, the Gulf of Guinea, the Carribean Sea, the Gulf of Mexico, the Norwegian Sea. The Indian Ocean is connected to the Arabian Sea and the Bay of Bengal. The Artic opens on the Barents Sea, the Greenland Sea and the Hudson Bay. The Southern Ocean’s main sea is the Weddell Sea.
Lakes, while embedded within landmasses can be saline or freshwater. The largest ones in Eurasia are the Caspian Sea, Lakes Baikal and Lagoda. In North America we find the Great Lakes: Superior, Erie, Michigan, Huron and Ontario, and many more. South America is home to Lake Titicaca. Lakes Victoria, Tanganyika and Malawi dominate their African brethren. Antarctica has one lake on this scale, Vostok, although it is currently enclosed below the ice sheet.
The mountains
Mountains are a very visible consequence of plate tectonics. Wherever continents collide, oceanic crust subducts or rift shoulders rise, the crust thickens and the topography reaches skyward.
The deep past is preserved through vast mountain ranges with eroded roots, now in orogenically inactive areas. The Scottish Highlands and Laurentian Mountains in Canada both share the same origin, more than one billion years ago, during the so-called Greenville orogeny. The Caledonian, Variscan, Appalachian and Uralian orogenies during the ensuing eons shaped the Norwegian fjords and Scandinavian mountain ranges, the Ural, Massif Central, Black Forest, and the Appalachians in America.
The planet’s tectonic activity today is focused on two giant tectonic belts. The Ring of Fire surrounds the Pacific plate. To the West its effect are mostly felt in terms of seismic and volcanic activity, and emerged mountain ranges are mostly present on islands like Japan. On the East side, it gave rise the to the Laramide and Andean orogenies, which respectively grew the Rocky Mountains range and the Andes.
The Alpide belt is a 15,000 kilometers long orogenic areas on the southern marches of Eurasia, tracing the closure of Thetys. It stretches from the Pyrénées, Alps, Atlas, Carpathians and Caucasus in the west, to the Alborz, Hindu Kush and Himalayas in the east.
Underwater relief
Mountains are the visible sculpture of continents, but the mid-ocean ridges are equally awe-inspiring, submerged reflief. They mark the place where oceanic crust is born, when the planet’s mantle rises beneath a rifted seam. It melts under the lower pressure, erupting as molten basalt that solidifies into a mountainous range. The Mid-Atlantic Ridge divides that ocean in two; the Southwest and Indian Ridges join three plates southeast of Africa, while the Pacific-Antarctic Ridge prolongs this system to the East and into the Souther Ocean. The East Pacific Rise, the fastest grower of all separates the vast Pacific plate from its neighbors.
Ridges and rises generate three square kilometres of new oceanic crust per year. Since the earth does not grow, that means some other part of the crust must disappear. That happens in subduction zones, where dense plates plunge back into the mantle. The steepest part of these descents are called trenches. The Mariana Trench, in the West Pacific is by far the deepest, falling almost eleven kilmetres below sea level, marking the deepest point on Earth. Also notable, the Tonga and Kermadec Trenches in the southwestern Pacific, the Philippine and Japan Trenches along the western Pacific arcs, the Kuril-Kamchatka and Aleutian Trenches at the Pacific’s northern rim, and the Peru-Chile Trench along South America’s western margin.
These structures reveal the underlying dynamics: the Pacific, surrounded by trenches, is shrinking; the Atlantic, with its central ridge and passive margins, is growing.
The ices
When snow falls faster than it can melt and accumulates over long periods, it is eventually compressed into dense ice, and becomes a permanent fixture (on human timescales). To remind ourselves of the proportions, only 2.5% of the earth’s water (hydrosphere) is freshwater. It exists in all three common states of matter:
atmospheric water vapour (often oscillating between a true gaseous state and a suspended liquid) constitutes a negligible amount, up to 0.03% of the total freshwater mass.
liquid surface water (lakes, rivers...) represents only 1% of the total.
liquid groundwater amounts to 30%.
the remaining 69% forms the cryosphere, the sum total of earth’s permanent ice and snow.
We currently live in an ice age. That figure has been higher in colder climates (up to 80% of freshwater being trapped in ice), but it has also been zero for most of our planet’s history.
99% of the cryosphere is concentrated in two vas ice sheets, the Antarctic ice sheet (covering 98% of that continent) and the Greenland ice sheet (over 80% of that island). Conventionally, smaller expanses of ice are called ice caps (the largest being 20,000 square kms). Of course, polar regions within the Arctic circle are sprinkled with them, in Russia, Norway, Iceland or Canada. To the South, they are present in Patagonia or New Zealand. But smaller ice caps can also exist anywhere at high enough altitudes, including close to the Equator, as can be observed in East Africa or in the Northern Andes... Sometimes, snow is not abundant enough to become ice, nor warm enough to melt, and persists as permanent snow.
Ice sheets and ice caps end up flowing slowly downslope under their own weight, from which point they are called glaciers. At a rate of only a couple meters per year, that flow is a powerful vector of glacial erosion, and a major actor in the creation of modern landscapes.
Under its own weight, the ice begins to flow slowly downslope or outward from an ice-sheet centre. Although this movement is often measured in only metres or tens of metres per year, glaciers have shaped our modern landscapes, making the result of their toil visible when they retreated.
The legacy of past glaciation include:
fjords: long, narrow, steep-sided inlets dug by glaciers below sea level, which were later flooded by the oceans.
cirques: bowl-shaped hollows where the glacier’s head used to be.
horns, like the famous Matterhorn.
drumlins: hills of glacial debris, whose long axes point the way the ice moved.
moraines: piles of rubbles dumped at a glacier’s edge, which can appear as proper landmasses today (Cape Cod, Long Island...).
lastly, deep basins carved by glaciers can trap the freshwater when the ice melts and become large lakes. The Great Lakes, which contain a fifth of today’s surface freshwater, are one example.
The rivers
As we have seen, surface freshwater is only a fraction of liquid freswhater (most of it is underground), a tiny fraction of all freshwater (most of which is ice), and an infinitesimal part of the earth water in general (most of which is saltwater).
A river system starts at sources or headwaters, where ice or snow melts, where atmospheric water condenses into rain, or where underground water emerges. Small streams combine to form larger channels known as tributaries, in turn feeding the main river stem. Rivers then descend from its sources to lower elevations, eroding, transporting and depositing sediment, all over the drainage basin (the area of land where all the running water is collected and drained into the river system). Finally the whole system converges to a single river and a mouth where it enters the sea or a lake .
In Africa, the principal river systems include the Nile, whose main river is the longest in the world by traditional measurement, draining northeastern Africa into the Mediterranean Sea. The Congo possesses the second-largest discharge and drains much of central equatorial Africa. Other major systems include the Niger, Zambezi, Orange, and Limpopo rivers.
In Europe, the broadest system is the Volga, which flows into the Caspian Sea. Other important rivers include the Danube, draining Central and Eastern Europe into the Black Sea; the Rhine, flowing into the North Sea; the Rhône, entering the Mediterranean; the Elbe, Vistula, Dnieper, Po, Loire, Seine, and Tagus.
Asia’s major rivers originate in the Tibetan Plateau. The Yangtze is the longest, followed by the Yellow River (Huang He), Mekong, Ganges, Indus, Tigris-Euphrates. Together they drain vast climatic zones ranging from tropical monsoon regions to Arctic tundra.
In North America, the dominant drainage network is the Mississippi-Missouri system, one of the largest on Earth. Other rivers include the Mackenzie and the Yukon (near the Arctic); the St. Lawrence, linking the Great Lakes to the Atlantic; the Columbia, Colorado and Rio Grande.
In South America, the Amazon is the world’s largest river by discharge, with the largest drainage basin on Earth. Other major systems include the Paraná-Paraguay-Río de la Plata network, the Orinoco, and the São Francisco.
Australia has one sizable river system, the Murray-Darling Basin.
The deserts
Like glaciation, aridity is a recurring climatic state. Modern deserts owe their existence to several mechanisms, sometimes in combination:
Subtropical belts, below thirty degrees in latitude are zones of descending dry air, and explain the largest deserts’ origins. The massive Sahara, which dwarfs all the others, the Arabian Desert, the Kalahari, much of the Australian interior, and the smaller deserts of the American Southwest.
Continentality parches wide areas that are too remote from the sea to experience much precipitation. It is experienced in the Gobi desert, the Taklamakan of China, and in Central Asia.
Rain shadows (cast by mountain ranges) block moisture-carrying winds and clouds, generating an "arid shadow". Typical examples include the Patagonian Desert in the lee of the southern Andes and parts of the Great Basin behind the Sierra Nevada.
Cold ocean currents suppress evaporation along tropical coasts and can produce hyper-arid environments like the Atacama of northern Chile or the foggy Namib.
Polar conditions, in the Arctic or Antarctica can also elicit dryness despite the presence of ice, owing to the low water-holding capacity of frigid air.
Volcanoes, hotspots, and the engines of catastrophe
Not all volcanism is the work of plate boundaries. Hotspots, regions of unusually voluminous volcanism rooted in mantle plumes that rise from deep within the Earth, build island chains, plateaus, and sometimes whole new sections of continent. As an oceanic plate drifts over a fixed plume, a chain of volcanoes is constructed, the youngest active over the hotspot and the older ones progressively extinct and subsiding. The principal hotspots include:
Hawaii — whose volcanic chain extends across the Pacific as the Hawaii-Emperor seamount chain, with a famous bend recording a major change in Pacific Plate motion around forty-seven million years ago.
Iceland — at the rare conjunction of a mid-ocean ridge and a hotspot, which is why it rises above the sea where the rest of the Mid-Atlantic Ridge is submerged.
Yellowstone — whose track can be followed northeast across the Snake River Plain in Idaho.
Réunion — the present terminus of a track that includes the Mascarene Plateau and, ultimately, the immense flood basalts of India’s Deccan Traps.
The Galápagos, the Canary Islands, Tristan da Cunha, and the Azores — each leaving its signature in volcanic rock and biological evolution.
The most spectacular volcanic events of all are the Large Igneous Provinces, or LIPs, in which immense volumes of basaltic magma erupt over geologically brief intervals — sometimes less than a million years:
The Siberian Traps (around 252 million years ago) — blanketed an area larger than western Europe in lava and gases; their eruption coincides almost exactly with the end-Permian mass extinction, the most severe biological catastrophe in Earth history, in which perhaps ninety-five per cent of marine species disappeared.
The Central Atlantic Magmatic Province (around 200 million years ago) — marks the rifting of Pangaea and coincides with the end-Triassic extinction.
The Deccan Traps (around 66 million years ago) — erupted around the Cretaceous-Palaeogene boundary, contributing along with the Chicxulub impact to the demise of the non-avian dinosaurs.
The Ontong Java Plateau — the largest LIP of all, a submarine plateau the size of Alaska in the western Pacific.
The Columbia River Basalts — the much younger flood basalts of the Pacific Northwest.
The Ethiopian Traps — accompanied the rifting of the Red Sea.
The North Atlantic Igneous Province — opened the seas between Greenland and Europe.
LIPs build landscapes — the stepped plateaux of the Deccan, the columnar basalts of the Columbia River Gorge, the lava plains of central Iceland — but they also kill. By injecting prodigious quantities of carbon dioxide and sulphur dioxide into the atmosphere, by acidifying the oceans, by depleting the ozone layer, they drive global crises of climate and chemistry that the biosphere takes millions of years to recover from. The correspondence between LIPs and mass extinctions is one of the sobering insights of modern earth science: catastrophe is not the exception in Earth history but a recurring movement in its symphony.